WO2016074949A1

WO2016074949A1 - Method for reducing the viscosity of viscosified fluids for applications in natural gas and oil fields

Info

Publication number: WO2016074949A1
Application number: PCT/EP2015/075247
Authority: WO
Inventors: Fatima DUGONJIC-BILIC; Jasmin MEURER; Marita Neuber
Original assignee: Tougas Oilfield Solutions Gmbh
Priority date: 2014-11-12
Filing date: 2015-10-30
Publication date: 2016-05-19

Abstract

The present invention relates to a method to reduce the viscosity of viscosified treatment fluids comprising a synthetic water soluble polymer, a breaker system containing an oxidative breaker and N-acyl or O-acyl-containing compound, mixing the viscosified treatment fluid and the breaker composition and allowing the viscosified treatment fluid and the breaker composition to interact whereby the viscosity of the viscosified treatment is reduced. The invention also relates to the application of the process in the production of oil and gas.

Description

Method for reducing the viscosity of viscosified fluids for applications in natural gas and oil fields

The present invention relates to a method to reduce the viscosity of viscosified treatment fluids comprising a synthetic water soluble polymer, a breaker system containing an oxidative breaker and N-acyl or O-acyl-containing compound, mixing the viscosified treatment fluid and the breaker composition and allowing the

viscosified treatment fluid and the breaker composition to interact whereby the viscosity of the viscosified treatment is reduced. The invention also relates to the application of the process in the production of oil and gas.

The resources for fossil fuels are highly exploited and also limited. With new and improved technologies these resources for oil and gas can be further exploited and unconventional reservoirs can be accessed. With the increasingly challenging conditions for the oil and gas production the requirements for the equipment and the chemicals also become more and more demanding.

Viscosified fluids are very common and widely used in almost all processes of the oil and gas production. They are applied for very different purposes and have to ensure various properties of the fluid:

• In drilling the bore hole a mud is circulated that lubricates and cools the drill head, disperses the cuttings and transports them upwards.

• In cementing the casing the fluid has to ensure a homogeneous transport of the cement particles downhole and to prevent sedimentation until the mixture is hardened.

• Fluids are viscosified to reduce friction caused by turbulent flow.

• Enhanced oil recovery of highly exploited formations can be achieved by flooding with viscosified water.

• Viscosified fluids are applied in water bearing formations to control the water and oil flow (relative permeability modifier)or to separate water rich area form oil containing area (water shut off)

• In well stimulation processes viscosified fluids transport solid particles (proppants) downhole into freshly created cracks in the formation. They prevent the

sedimentation of the proppants and possibly blockade of the bore hole.

• Viscosified acids are also used in well stimulation processes. So it is ensured that the acid reacts slowly and can reach the desired regions especially in horizontal wells. Water based fluids are typically made viscous by the addition of water soluble polymers. The polymers can be natural based polymers or synthetic polymers.

The polymers fulfil various tasks. Amongst others they function as rheology modifier, fluid loss additive, friction reducer, viscosifier or precursor for crosslinked hydrogels.

As natural based polymers typically polysaccharides or modified polysaccharides are used. For instance, suitable hydratable polysaccharides include starch or its derivatives, galactomannan gums, glucomannan gums, cellulosic derivatives, preferably carboxymethyl cellulose; cellulose ether, preferably hydroxyethyl cellulose; guar gums or its derivatives, preferably hydroxyalkyl guar, carboxyalkyl guar, and carboxyalkyl hydroxyalkyl guar or hydrophobically modified guar alginates, carrageenans, tragacanth gums, glucan gums and xanthan gums.

All the natural based polymers have in common that due to their glycosidic bonds they are not stable at elevated temperatures and under acidic conditions. They are also easily attacked and degraded by bacteria and their dispersions and solutions have to be protected by biocides even when they are handles or stored only for short periodes of time. Furthermore the availability of natural based polymers may depend on weather conditions and crop yield. This depencence impacts the availability and the price of natural based polymers.

Therefore often synthetic polymers based on acrylamide are used for applications in the oil and gas production as substitutes for natural based polymers. They are independent from bad weather conditions and distinguish themselves with marked better temperature and chemical stability and resistance against bacterial

degradation.

Acrylamide can be copolymerized with a broad variety of monomers to adjust the properties of the resulting water soluble polymer. Amongst others, ethylenically unsaturated carboxylic, sulfonic or phosphonic acids, their esters, unsubstituted or N- and Ν,Ν-substituted derivatives of amides of ethylenically unsaturated carboxylic acids, N-substituted (cyclic) derivatives of ethylenically unsaturated amides can be used.

The viscosity of fluids containing polymers as thickener can be further increased by crosslinking the polymer chains to form a hydrogel, that is a three dimensional network of extremely high molecular weight. Typically, polyvalent cations of group IMA, IVB, VB, VIB, VIIB and/or VI I IB of the periodic table of the elements are used as crosslinking compound, preferred are compounds of boron, zirconium, titanium, aluminum or chromium, for both synthetic polymers and natural polymers.

The viscosity of the viscosified fluids or of the crosslinked hydrogels may range from almost as thin as water (1 mPas) to several 1000 mPas.

For some applications it is necessary to reduce the viscosity of the fluid after a desired time. Especially in well stimulation processes the viscosity of the frac fluid is reduced to ensure the removal of the fluid from the bore hole to prevent blockage of the pores and fractures by the highly viscos gel and to minimize formation damage.

For the reduction of the viscosity of the fluid typically the water soluble polymer is degraded by so called breakers. The breakers cleave the high molecular polymer chain into fragments of lower molecular weight.

Solutions of high molecular weight polymers show a high viscosity. Cleaving bonds in the polymer chain gives fragments of lower molecular weight; the viscosity of their solution is reduced. The viscosity of polymer solutions is therefore related to the molecular weight of the polymer.

Analytically the degradation of the polymer chain or network can be characterized by rheological methods in determining the viscosity of the polymer solution. As polymer solutions typically are non-Newtonian fluids it is important to compare the results only when the methods for the determination of the viscosity were exactly the same.

The polymer degradation can be followed either directly in a rheometer or by

determination of the viscosity of the polymer solution before and after breaking treatment.

Typically, oxidizers, enzymes or acids are used as breakers.

Enzymes are generally used for naturals based polymers in a low temperature range from about 20 °C to 70 °C. At higher temperature they denature and become ineffective. Enzymes break selectively glycosidic linkages in polysaccharides, so very specific types of enzymes are used for different types of naturals based polymers. At lower

temperature the reaction rate becomes very slow. Acids also break the glycosidic linkage by hydrolysis. However the breaking process is difficult to control, the chemical compatibility with the viscosified fluid and its additives is difficult and corrosion of the metal equipment may occur.

Synthetic polymers, especially polyacrylamide based polymers are much more stable compared to natural based polymers. Polyacrylamide based polymers are considered to be very resistant towards thermal, acidic, enzymatic or biological degradation. Therefore they typically are used at high temperatures or otherwise harsh conditions. Under harsh conditions oxidative breakers are able to degrade also synthetic polymers.

The use of oxidative breakers is described in the literature for natural based polymers as well as for synthetic, polyacrylamide based polymers. The oxidative breakers have in common that they are able to release hydrogen peroxide that is a strong oxidizing agent. The polymer chain is degraded by oxidative processes.

Typically persulfates are used in the low temperature range up to about 100°C (212°F). Sodium persulfate, ammonium persulfate, potassium persulfate are by far the most common oxidative breakers. They are effective in the temperature range that is also ideal for the application of natural based polymers as guar or hydroxypropyl guar for hydraulic fracturing.

At higher temperatures persulfates decompose too fast. At temperatures above 100°C (212°F) often bromates are applied. Their use is also described for synthetic polymers at very high temperatures above 150 °C (302 °F). Typical operation conditions of breakers are given for example in G.A. Al-Muntasheri, SPE Wetern North American and Rocky Mountain Joint Regional Meeting, Denver, Colorado, USA, 16-18 April 2014,

SPE169552.

For every application the breaker must meet the specific needs. In hydraulic fracturing processes it is important that in the beginning the viscosity of the fluid is high and remains stable for a given time to ensure a proper transport of the proppants into the freshly created fractures. Only after proppant placement the viscosity of the frac fluid should decrease. It is desirable that the viscosity of the fluid then reaches values similar to water in a short time. Furthermore no residues should be left over that could block the fractures and diminish the flow of oil or gas. The effectivity of every substance that is used as a breaker depends strongly on the conditions, especially on temperature, pH-value and concentrations of the breaker itself as well as of the polymer. Some of the conditions are given by the application and cannot be changed, for example the borehole temperature. Some conditions can be altered at least within certain limits to ensure an effective degradation of the polymers, for example the breaker concentration or the well closure time.

Especially the temperature is important for the breaker effectivity. The rate at which a persulfate molecule breaks into two radicals is temperature dependent. Typically, below 120°F (49°C) this process occurs very slow, and the reaction must be catalysed to obtain acceptable break rates. A variety of catalysts, including various organic amines and inorganic materials, may be used for persulfate breakers. The optimum pH value for persulfate oxidation is around pH 10 at low temperatures (less than 150°F or 66°C). Above approximately 200°F (93°C), persulfate decomposes fast and cleaves the polymer chains very quickly (i.e. without delay in the breaking process). Therefore, persulfate is typically not recommended as breaker above 200°F (93°C).

Therefore, breakers and the breaking conditions must be selected carefully. Premature reduction of viscosity during pumping of the frac fluid can jeopardize the treatment.

Width and length of the fractures is influenced in unknown extent and the proppant placement within the fractures is not ensured. On the other hand, inadequate reduction of the fluid viscosity after the pumping can also lead to severe reduction of oil and gas flow due to resistance of the viscous fluid.

The effect of breakers can be adjusted. For higher temperatures breakers can be encapsulated which allows release only after some time and early polymer degradation is prevented thereby.

For lower temperatures the use of activators is reported in the literature.

It is known from bleaching in laundry and cleaning processes that activators for bleaching agents, e.g. sodium perborate and sodium percarbonate can be used to enable an effective process at low temperatures. Typical activators are mainly N- and O- acyl compounds. They are able to react with hydrogen peroxide that is released from the bleaching agents and form very active peracids.

The most common activator is tetraacetyl ethylendiamine (TAED) that reacts with hydrogen peroxide from the bleaching agent to form peracetic acid. Nonanoyloxy benzenesulfonate (NOBS), dodecanoyl oxybenzenesulfonate (LOBS) or decanoyl oxybenzoic acid (DOBA) allow even lower temperatures. A great variety of N- and O- acyl compounds is claimed as activator for persalts in the open and patent literature, e.g. U.S. Patent No. 4,412,934 or G. Reinhardt, 5th World Conference on Detergents, edit. A. Cahn, AOCS Publishing, 2003, p. 69 to 74.

Activators are also described for oxidative breakers for processes in the oil and gas production.

U.S. Patent No. 7,21 6,704 describes the use of a reducing sugar as activator for oxidative breaker. Further addition of iron salt enhances the breaking of the polymer. The activator is claimed to be useful for temperature below 121 °C (250 °F).

U.S. Patent No. 7,923,417 claims a method for degrading xanthan at temperatures below 38 °C (100 °F) by using a breaker system comprising a source of hydrogen peroxide and a water soluble alkanoyl donor compound. The activator is selected from the group consisting of ethyl acetate, ethyl acetoacetate, tetraacetylethylendiamin, sodium nonanoyloxybenzenesulfonate and their combinations.

U.S. Patent No. 8,415,278 also describes a breaker system for xanthan, diutan and succinogycan gums comprising a source of hydrogen peroxide and a water soluble alkanoyl donor compound. The activator is selected from tetraacetylethylendiamin, sodium nonanoyloxybenzenesulfonate and their combinations. The method is useful for applications when the temperature of the formation is less than 38°C (100°F).

U.S. Patent No. 7,928,040 claims a method for degrading xanthan at temperatures below 38°C (100°F) by using a breaker system comprising a source of hydrogen peroxide and a chelated transition metal, a chelating agent and a transition metal and their combinations. The chelation agent comprises EDTA, alkali metal salt of EDTA, alkali metal citrate, citric acid, iminodiacetic acid, gluconate, NTS, tetraamidomacrocyclic ligand or their combinations.

U.S. Patent Application No. US 2009/0221453 describes a beaker system for hydratable polymers comprising an oxidizing breaking system and an activator capable of providing a bisulfite ion source. The temperature range for the methods applying the breaker system is from 25°C to 177°C. There are various methods described that show the activation of breaker substances for natural based polymers. However there is a need for easy breaking at low temperatures for the thermal and chemical much more stable synthetic polymers. The range of applications for this polymer class could widely be extended.

Surprisingly it was found that by the addition of N- and O-acyl compounds the effect of oxidative breakers for very stable synthetic polyacrylamide based polymers could be increased.

Therefore the present invention relates to a method of reducing the viscosity of a viscosified treatment fluid comprising the measures:

(i) providing a viscosified treatment fluid containing at least a synthetic, water

soluble gelling agent,

(ii) providing a breaker composition containing at least one oxidative breaker and at least one N-acyl or O-acyl-containing compound,

(iii) mixing said viscosified treatment fluid and said breaker composition,

(iv) allowing said viscosified treatment fluid and said breaker composition to interact whereby the viscosity of the viscosified treatment is reduced.

According to the invention the effect of an oxidative breaker is significantly increased when at least one N- acyl or O-acyl compound is added to the oxidative breaker. The quantity of oxidative breaker therefore can be reduced. It is of special advantage that either the degradation of a synthetic polyacrylamide based polymer can be accelerated at a given temperature or that the temperature for the breaking of a synthetic

polyacrylamide based polymer can be lowered.

Synthetic water soluble gelling agent

According to the instant invention, the synthetic water soluble gelling agent is a water soluble synthetic polymer material, in particular such synthetic polymer material are synthetic polyacrylamide based polymers, copolymers or terpolymers.

The polymer used in the method of this invention is a synthetic polymer comprising: (I) at least structural units of formula (I)

(I)

wherein

R1 , R2 and R3 independently are hydrogen or d-Ce-alkyl,

from 0 to 95 % by weight structural units of formula (II)

R4

-CH;

0=S=0

I

0-R5 wherein

R4 is hydrogen or Ci-Ce-alkyl,

R5 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

A is a covalent C-S bond or a two-valent organic bridging group, from 0 to 95 % by weight structural units of formula

R6 R7

I

^■CH-

0-R8

(I I I)

wherein

R6 and R7 are independently of one another hydrogen, d-Ce-alkyl,

-COOR9 or -CH2-COOR9, with R9 being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R8 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine, or is d-Ce- alkyl, a group -Cnh OH with n being an integer between 2 and 6, preferably 2, or is a group -C0H20-NRI 0R1 1 , with 0 being an integer between 2 and 6, preferably 2, and

R10 and R1 1 are independently of one another hydrogen or d-Ce-alkyl, preferably hydrogen,

(IV) from 0 to 95 % by weight structural units of formula (IV)

R12 R13

R14 Λ R15

(IV)

wherein

R12 and R13 are independently of one another hydrogen, d-Ce-alkyl,

-COOR1 6 or -CH2-COORI 6, with

R1 6 being hydrogen, a cation of an alkaline metal, of an earth

alkaline metal, of ammonia and/or of an organic amine, R14 is hydrogen or, d-Ce-alkyl, and

R15 is -COH, -CO-Ci-Ce-alkyl or

R14 and R15 together with the nitrogen atom to which they are attached form a heterocyclic group with 4 to 6 ring atoms, preferably a pyridine ring, a pyrrolidone ring or a caprolactame ring, (V) from 0 to 20 % by weight structural units of formula (V)

R1 7

— CH

B

I

15 I

0-R18

(V)

wherein

R17 is hydrogen or, d-Ce-alkyl, and

R18 and R19 are independently of one another hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

B is a covalent C-P bond or a two-valent organic bridging group, with the proviso that the percentage of the structural units of formulae (I) to (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I) to (V) amounts to 100%. The polymer of the method of this invention may further contain crosslinking monomers, which are monomers with more than one ethylenically unsaturated group. Different compound classes can be used, such as bis-amides, e.g. methylene-bis-acrylamide, bis-, tris- or tetraether derived from two-, three- or fourvalent alcohols and from ethylenically unsaturated halides e.g. trimethylolpropane diallylether, pentaerithriol- triallylether and tetrallyloxyethane, or esters of ethylenically unsaturated carboxylic acids with multivalent alcohol, e.g. di-, tri-, or tetraacrylates derived from

ethyleneglycol, from trimethylolpropanol or from pentaerythrite, or di-, tri-, or polyamines which are substituted at the nitrogen atom with ethylenically unsaturated residues, such as Ν,Ν'-diallyl-ethylenediamine or triallylamine.

Crosslinker monomers, if present, typically are used in amounts between 0.01 and 2 % by weight, preferably between 0.05 and 1 % by weight, referring to the total amount of monomers used.

The C-i-Ce-alkyl groups being present may be straight-chain or branched. Examples of alkyl groups are methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, tert.-butyl, n- pentyl or n-hexyl. Ethyl and especially methyl are preferred.

The group A may be a C-S-covalent bond or a two-valent organic group. Examples thereof are d-Ce-alkylene groups or -CO-d-Ce-alkylene groups. The alkylene groups may be straight-chain or branched. Examples of A groups are -C_PH2_P- groups or -CO-NH-C_PH2_P- groups, with p being an integer between 1 and 6. -CO- NH-C(CH3)2-CH2- or a C-S-covalent bond is a preferred group A.

The group B may be a C-P-covalent bond or a two-valent organic group. Examples thereof are d-Ce-alkylene groups. These groups may be straight-chain or branched. Examples of alkylene groups are -Cqh q- groups, with q being an integer between 1 and 6. Methylene or a C-P-covalent bond is a preferred group A.

The structural units of formula (I) are derived from an ethylenically unsaturated carboxylic acid amide selected from the group of acrylamide, methacrylamide and/or their N-d-Ce-alkyl derivatives or N,N-Ci-C6-dialkyl derivatives.

Preferred polymers used in the method of this invention further contain structural units of formula (II) to (V) which are derived from an ethylenically unsaturated sulfonic acid and/or its alkaline metal salts and /or their ammonium salts, from ethylenically unsaturated carboxylic acid and/or its alkaline metal salts and /or their ammonium salts, from N-vinylamides, and/or an ethylenically unsaturated phosphonic acid and/or its alkaline metal salts and /or their ammonium salts, optionally together with further copolymerisable monomers.

Other preferred copolymers used in the method of this invention are those, wherein B is a C-P covalent bond or a -Cqhbq- group with q being an integer between 1 and 6, preferably 1 , and/or wherein A is a C-S covalent bond or a -CO— NH-Cphbp- group with p being an integer between 1 and 6, preferably between 2 and 4, B being most preferably a group -CO-NH-C(CH₃)2-CH₂-.

Also preferably applied are copolymers with structural units of the formula (II) derived from vinylsulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, 2- methacrylamido-2-methylpropane sulfonic acid, styrene sulfonic acid and/or their alkaline metal salts and/or their ammonium salts. Especially preferred are structural units of the formula (II) derived from 2-acrylamido-2-methylpropane sulfonic acid and/or from 2-methacrylamido-2-methylpropane sulfonic acid and/or from their alkaline metal salts and/or from their ammonium salts.

Further preferably applied monomers which are optionally used in the manufacture of the copolymers are chosen from ethylenically unsaturated carboxylic acid and/or their derivatives of the formula (III), preferably chosen from the group of alkylesters from ethylenically unsaturated carboxylic acid, oxyalkyl esters of ethylenically unsaturated carboxylic acid and/or esters of ethylenically unsaturated carboxylic acids with N- dialkylalkanolamines.

The ethylenically unsaturated carboxylic acids of the formula (III) are preferably acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid and/or crotonic acid as well as their alkaline metal salts and/or their ammonium salts. The alkylesters of ethylenically unsaturated carboxylic acids are preferably alkylesters of acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid and/or crotonic acid.

Especially preferred are alkylesters with 1 to 6 carbon atoms.

The oxyalkyl esters of an ethylenically unsaturated carboxylic acids of the formula (III) are preferably 2-hydroxyethylester of acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid and/or crotonic acid.

The ester of ethylenically unsaturated carboxylic acid of the formula (III) with N- dialkylalkanolamine is preferably Ν,Ν-dimethylethanolamine methacrylate, its salt or quaternary ammonium product. Further preferably applied copolymers with structural units of the formula (IV) are derived from N-vinylamides.

The N-vinylamide is preferably N-vinylformamide, N-vinylacetamide, N-vinyl-N- methylacetamide, or N-vinylamide comprising cyclic N-vinylamide groups, preferably derived from N-vinylpyrrolidone, N-vinylcaprolactame or N-vinylpyridine.

Preferably applied are copolymers with structural units of the formula (V) are derived from vinylphosphonic acid and/or its alkaline metal salts and/or its ammonium salts, and/or allylphosphonic acid and/or its alkaline metal salts and/or its ammonium salts.

Preferred copolymers used in the method of this invention are those, wherein Ri, R2, P»3, P»4, R10, R11 , Ri4, and R17 are independently of one another hydrogen or methyl or wherein R5, Rg, Rie, Ris and R19 are independently of one another hydrogen or a cation of an alkali metal, of an earth alkaline metal, of ammonia or of an organic amine.

Still other preferred copolymers used in the method of this invention are those, wherein Re and R12 is hydrogen and R7 and R13IS hydrogen or methyl, or wherein R6 is -COOR9 and R7 is hydrogen or wherein Re is hydrogen and R7 is -CH2-COOR9 or wherein R12 is hydrogen and R13 is hydrogen or methyl, or wherein R12 is -COOR16 and Ri3 is hydrogen or wherein R12 is hydrogen and R13 is -CH2-COOR16.

The copolymer of the method of this invention may be linear or branched or crosslinked either by covalent or ionic crosslinking.

The polymers can be synthesized by various technologies, e.g. by inverse emulsion polymeriziation, gel polymerizaiton or precipitation polymerization.

A viscosified treatment fluid is prepared by dissolving a solid polymer or by diluting a polymer solution or by inverting a water-in-oil polymer emulsion using water or an aqueous solution.

The aqueous solution may contain salts such as NaCI, KCI,CaCl2, MgCl2, NH₄CI, NaBr, KBr, sodium formate, sodium acetate, and others.

The aqueous solution may further contain water miscible solvent as alcohols, e.g. methanol, ethanol, n- and i-propanol, glycol. The aqueous polymer solution may further contain additives that are necessary for the treatment. Those additives may include surfactants, biocides, corrosion inhibitors.

The concentration of the polymer is typically from 0.05 to 10 % by weight, preferred from 0.1 to 5 % by weight and most preferred from 0.2 to 2 % by weight, referred to the aqueous polymer solution.

To increase the viscosity of the treatment fluid, the polymers may also be ionically crosslinked by multivalent metal ions or metal complexes selected from group IIIA, IVB, VB, VIB, IIVB and/or VI 11 B of the periodic table of elements, preferably selected form the ions and/or complexes of zirconium, aluminium, titanium, boron, chromium and/or iron. Especially preferred are the ions and/or complexes of zirconium and titanium.

Typically water soluble salts of the multivalent metal ions are used. Suitable anions are e.g. halides, especially chloride, sulfate, lactate, citrate or gluconate. Also suitable are complexes of the multivalent metal ions with organic N- and O-compound, e.g. alcohols, di- and triols, mono-, di- and tri- carboxylic acids, mono-, di- and triamines and/or hydroxyalkylamines.

The quantity of transition metal compound for crosslinking the polymers ranges 0.1 to 50 % by weight, preferred from 0.5 to 30 %, more preferred from 1 to 20 % by weight, referred to the total mass of polymer.

The transition metal compounds, e.g. the salts and/or complexes of transition metal cation, are dissolved and/or diluted in water or in a water miscible solvent, and then added to the polymer solution under stirring to ensure a homogenous distribution of transition metal cation in the solution. The crosslinking of the polymer chains can be retarded or speeded up by adaptation of the stirring speed, pH value and/or adjusting the temperature.

The viscosity of the viscosified fluids or of the crosslinked hydrogels prior to braking typically may range from about 5 mPas to several 1000 mPas.

Breaker composition

According to the instant invention, the breaker composition contains at least one oxidative breaker and at least one N-acyl or O-acyl-containing compound. The breaker composition of the instant invention containing at least one oxidative breaker and at least one N-acyl or O-acyl-containing compound is typically provided as a dissolved solution, mixed powder or a dispersion.

Typical concentrations of the oxidative breakers are in the range from 0.1 to 100 % by weight, preferred from 0.5 to 50 % by weight, referred to the polymer mass.

Typical concentrations of the N-acyl and/or O-acyl activator compounds are in the range from 0.1 to 100 % by weight, preferred from 0.5 to 50 % by weight, referred to the mass polymer.

The concentration of the breaker composition is typically from 0.2 to 100 % by weight, preferred from 0.5 to 50 % by weight and most preferred from 0.5 to 10 % by weight, referred to the aqueous polymer solution.

Typical the mass ratio of the oxidative breakers to N-acyl and/or O-acyl activator compounds is from 0.01 to 1 to 100 to 1 .

Oxidative breaker

According to the instant invention, suitable oxidative breakers are inorganic and organic compounds that release or form hydrogen peroxide upon dissolution in water. This includes, but is not necessarily restricted to, inorganic persalts, e.g., persulfates, perborates, percarbonates and/or perphosphates. Preferred inorganic persalts are sodium, potassium and ammonium persulfate, sodium perborate monohydrate, sodium perborate tetrahydrate, sodium carbonate and their

combinations.

Further suitable oxidative breakers of the method of this invention include, but are not necessarily restricted to, organic peroxides. Preferred organic peroxides are t- butyl peroxypivalat, dilauroyl peroxide, diacetyl peroxydicarbonate , dibenzoyl peroxide, t-amyl peroxybenzoate.

Oxidative breakers also include hydrogen peroxide itself and its adducts such as hydrogen peroxide urea.

N-acyl or O-acyl-containing compound

According to the instant invention, activators for oxidative breakers are N- and O-acyl compounds which are able to react with hydrogen peroxide that is released from oxidative breaker and to form peracids. It is known from bleaching in laundry processes that the activators become effective even at low temperature, e.g. at room temperature (25°C and lower).

Suitable activators of the method of this invention include, but are not necessarily restricted to tetraacetyl ethylendiamine (TAED), tetraacetyl glycoluril, Ν,Ν'-diacetyl hydantoin and/or 1 ,5-diacetyl-2,4-dioxo hexahydro-1 ,3,5-triazine (DADHT).

Further suitable activators are acyloxy benzene sulfonic acids and their alkali metal salts, such as nonanoyloxy benzene sulfonic acid (NOBS), dodecanoyl oxybenzene sulfonic acid, benzoyloxy benzene sulfonic acid and/or decanoyl oxybenzoic acid and their alkali metal salts.

It is known that the rate of peracid formation is strongly dependent on the pH. It may therefore become necessary to keep the pH value in the optimum range for obtaining an effective break and one way of doing this is to use a pH buffer as additional ingredient.

Mixing of viscosified treatment fluid and breaker composition

Typically the treatment fluid can be prepared by adding breaker and activator and if wanted the crosslinker or their solutions to the solution of the gelling agent, such as synthetic polymer material, under stirring to ensure a substantially homogeneous distribution of all ingredients within the treatment fluid. If necessary further additives can be present or can be added e.g. pH buffer material or other substances to adjust the pH value. The so prepared treatment fluid is then pumped into the formation.

The sequence of the addition of the crosslinker solution, the oxidative breaker or its solution, the activator or its solutions and, if necessary, further solutions of additives to the polymer solution can be varied. It is also possible to add them all

simultaneously, e.g. continuously during pumping of the polymer solution into the formation.

It is preferred to add first the breaker and the activator or their solutions, respectively, to the aqueous polymer solution and then add the crosslinker solution and/or solutions of additives if necessary directly before introducing the treatment fluid into the formation. For some applications the breaker and activator solution can be added to the polymer solution or the polymer gel when the solution or the gel have already been introduced into the formation and, if required, have already stayed there for a given time.

After some time of close contact of the gelling agent, such as the polymer, and the breaker and activation system the polymer is degraded and the viscosity of the treatment fluid is reduced. The reduction of the viscosity is dependent on the conditions in the formation, e.g. temperature, pH, and concentrations of polymer, oxidative breaker and activator and of the time. It may depend on the application what degree of reduction of the viscosity is required.

A reduction of the viscosity of the treatment fluid by means of the activated breaker system is considered to be achieved (within the meaning of the instant invention) when within 1 hour a viscosity of less than 90 % compared to the original viscosity is obtained, the viscosities determined under the exact experimental conditions. Thus, the interaction of viscosified treatment fluid and breaker composition causes a reduction of the viscosity of at least 10 % within 60 minutes (determined as described before, such as exact experimental conditions).

The treatment fluid according to the invention is preferably injected into the formation at a temperature below 90°C, more preferred below 75°C.

Test Methods

The following testing methods are used:

Viscosity Viscosity was determined at 30°C using capillary viscosimetry

(UBBELOHDE). The capillary with the appropriate diameter for the viscosity of the fluid was chosen.

Abbreviations

HLB HLB-value means the hydrophilic-lipophilic balance of a surfactant and is a measure of the degree to which it is hydrophilic or lipophilic, determined by calculating values for the different regions of the molecule. There are different methods to calculate the HLB-value. The most common results in a ranking of the surfactants between 0 and 20 with 0 corresponds to a completely

lipophilic/hydrophobic molecule, and a value of 20 corresponds to a completely hydrophilic/lipophobic molecule. Typically the suppliers specifies the HLB-value of the surfactant. TDS TDS means total dissolved solids. The concentration is typically given in g/kg or mg/kg = ppm.

The following examples illustrate the invention without limiting it. Examples:

Example 1 : Preparation of a polymer via inverse emulsion polymerization

37 g sorbitan monooleate were dissolved in 160 g Cn-Gie isoparaffin. 100 g water in a beaker were cooled to 5 °C, then 50 g 2-acrylamido-2-methylpropane sulfonic acid and 10 g vinylphosphonic acid were added. The pH was adjusted to 7.1 with aqueous ammonia solution. Subsequently 223 g acryl amide solution (60 weight% in water) were added.

Under vigorous stirring the aqueous monomer solution was added to the isoparaffin mixture. The emulsion was then purged for 45 min with nitrogen.

The polymerization was started by addition of 0.5 g azoisobutyronitrile in 12 g isoparaffin and heated to 50°C. To complete the reaction the temperature was increased to 80°C and maintained at this temperature for 2 hours. The polymer emulsion was cooled to room temperature. As product, a viscous fluid was obtained.

Example 2: Preparation of a polymer via gel polymerization

400 ml deionized water and 9.2 ml 25 weight-% aqueous ammonia solution were placed in a reaction vessel. 70 g acryl amide and 30 g acrylic acid were added under stirring. The solution was purged with nitrogen and heated to 50°C. The

polymerization was started by addition of 5 ml of a 20 % by weight aqueous solution of ammonium persulfate. To complete the reaction the temperature was increased to 80 °C and maintained at this temperature for 2 hours. After cooling to room

temperature a highly viscous gel was obtained.

Example 3:

A synthetic brine was prepared from tab water, CaCl2^*2H20, NaCI, KCI and

MgCl2^*6H₂0. The concentrations were 13,000 ppm Ca, 9,700 ppm Na, 200 ppm K, 100 ppm Mg and 38,400 ppm CI, the overall TDS being 61 ,400 ppm. 200 g of the synthetic brine and 2 g of a non-ionic surfactant with a HLB of about 13 were mixed in a Waring blender. 3.5 g of the polymer emulsion of example 1 were added and for 4 min. Then breaker or breaker system was added, the quantities given in table 1 . Next 0.5 g acetic acid and 1 .5 g zirconium (IV) triethanol amine solution (25 weight% in water) were added. A gel is formed within few seconds and can easily be lifted with a glass rod from the blender. The initial gel viscosity is > 1000 mPas at 30°C (the gel could not be placed into the capillary appropriate for up to 1000 mPas).

The gel was placed in a glass bottle, sealed and put into a water bath at 70°C without stirring. After 2 hours the bottle was allowed to cool to room temperature and the viscosity of the broken fluid was determined at 30°C using capillary viscosimetry.

The quantities of the breaker and breaker system are given in table 1 as well as the results of the breaking experiments.

Table 1

It is obvious that the addition of TAED to the oxidative breaker to form the breaker system enhances the degradation of the polymer chain and leads to broken fluid with very low residual viscosity.

Example 4:

200 g of tab water, 2 g KCI and 2 g of a non-ionic surfactant with a HLB of about 13 were mixed in a Waring blender. 2.5 g of the polymer emulsion of example 1 were added and for 4 min. Then breaker or breaker system was added, the quantities given in table 1 . Next 0.5 g acetic acid and 0.75 g zirconium (IV) triethanol amine solution (25 weight% in water) were added. A gel is formed within few seconds and can easily be lifted with a glass rod from the blender. The initial gel viscosity is > 1000 mPas at 30°C (the gel could not be placed into the capillary appropriate for up to 1000 mPas). The gel was placed in a glass bottle, sealed and put into a water bath at 60°C without stirring. After 2 hours the bottle was allowed to cool to room temperature and the viscosity of the broken fluid was determined at 30°C using capillary viscosimetry.

The quantities of the breaker and breaker system are given in table 2 as well as the results of the breaking experiments.

Table 2

Example 5:

182 g of tab water and 14 g KCI were mixed in a Waring blender. 4 g of the polymer gel of example 2 were added and stirred with low speed for several minutes until the gel is dissolved completely. The viscosity of the polymer solution was 95 mPas at 30 °C.

Then breaker or breaker system was added, see table 3. The mixture was stirred for one minute, then the polymer solution was placed in a glass bottle, sealed and put into a water bath at 60°C without stirring. After 1 hour the bottle was allowed to cool down and the viscosity of the broken fluid was determined at 30°C using capillary viscosimetry.

Table 3

Again the breaker system comprising an oxidative breaker and a N-Acyl compound significantly enhances the degradation of synthetic polymer.

Example 6:

200 g of the synthetic brine described in example 3 and 2 g of a non-ionic surfactant with a HLB of about 13 were mixed in a Waring blender. 3.5 g of the polymer emulsion of example 1 were added and for 4 min. Then breaker or breaker system was added, the quantities given in table 4. Next NaOH solution was added to get a pH of 10, then 1 .5 g zirconium (IV) lactat solution (25 weight% in water) were added. A very strong gel is formed within few seconds and can easily be lifted with a glass rod from the blender. The initial gel viscosity is > 1000 mPas at 30°C (the gel could not be placed into the capillary appropriate for up to 1000 mPas).

The gel was placed in a glass bottle, sealed and put into a water bath at 60°C without stirring. After 2 hours the bottle was allowed to cool down and the viscosity of the broken fluid was determined at 30°C using capillary viscosimetry.

Table 4

The results show that the breaker system works effectively in a very broad pH range. A very strong crosslinked polymer gel can be degraded at low temperature to obtain a broken fluid with low viscosity.

Claims

1 . Method of reducing the viscosity of a viscosified treatment fluid comprising the measures:

(i) providing a viscosified treatment fluid containing at least a synthetic water soluble gelling agent,

(iii) mixing said viscosified treatment fluid and said breaker composition,

(iv) allowing said viscosified treatment fluid and said breaker composition to interact whereby the viscosity of the viscosified treatment fluid is reduced.

The method as claimed in claim 1 , wherein the synthetic water soluble gelling agent is a water soluble synthetic polymer material, in particular such synthetic polymer material are synthetic polyacrylamide based polymers, copolymers or terpolymers.

The method as claimed in claim 2, wherein the water soluble synthetic polymer material is a synthetic polymer comprising:

(I) at least structural units of formula (I)

(I)

wherein

R1 , R2 and R3 independently are hydrogen or d-Ce-alkyl,

from 0 to 95 % by weight structural units of formula (II)

R4

CH

A

I

o=s=o

I

0-R5

(II) wherein

R4 is hydrogen or d-Ce-alkyl,

A is a covalent C-S bond or a two-valent organic bridging group, from 0 to 95 % by weight structural units of formula (III)

R6 R7

0-R8

(III)

wherein

R6 and R7 are independently of one another hydrogen, d-Ce-alkyl,

-COORg or -CH2-COORg, with Rg being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R8 is hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine, or is d-Ce- alkyl, a group -Cnh OH with n being an integer between 2 and 6, preferably 2, or is a group -C0H20-NRI 0R1 1 , with 0 being an integer between 2 and 6, preferably 2, and R10 and R1 1 are independently of one another hydrogen or d-Ce- alkyl, preferably hydrogen,

(IV) from 0 to 95 % by weight structural units of formula (IV)

R12 R13

r I -1

CH

R14 Λ R15 (IV)

wherein

R12 and R13 are independently of one another hydrogen, Ci-Ce-alkyl,

-COOR16 or -CH2-COORie, with Rie being hydrogen, a cation of an alkaline metal, of an earth alkaline metal, of ammonia and/or of an organic amine,

R14 is hydrogen or, Ci-Ce-alkyl, and

R15 is -COH, -CO-Ci-Ce-alkyl or R14 and R15 together with the nitrogen atom to which they are attached form a heterocyclic group with 4 to 6 ring atoms, preferably a pyridine ring, a pyrrolidone ring or a caprolactame ring, (V) from 0 to 20 % by weight structural units of formula (V)

R1 7

CH^

B

I

15 I

0-R18

(V)

wherein

R17 is hydrogen or, d-Ce-alkyl, and

B is a covalent C-P bond or a two-valent organic bridging

group,

with the proviso that the percentage of the structural units of formulae (I) to (V) refer to the total mass of the copolymer and the percentage of the structural units of formulae (I) to (V) amounts to 100%.

The method as claimed in claim 2 or 3, wherein water soluble synthetic polymer material contains crosslinking monomers, which are monomers with more than one ethylenically unsaturated group.

The method as claimed in claim 2, 3 or 4, wherein the water soluble synthetic polymer material is selected from the group consisting of polymers containing:

(i) 10 to 90 % by weight of structural formula I, preferred from 20 to 70 % by weight,

(ii) 0 to 80 % by weight of structural formula II, preferred from 10 to 60 % by weight,

(iii) 0 to 50 % by weight of structural formula III, preferred from 0 to 40 % by weight,

(iv) 0 to 50 % by weight of structural formula IV, preferred from 0 to 40 % by weight,

(v) 0 to 20 % by weight of structural formula V, preferred from 0 to 5 % by

weight, referred to the total mass of the polymer.

6. The method as claimed in one or more of claims 1 to 5, wherein the oxidative breaker is selected from the group consisting inorganic persalts, preferred inorganic anionic persalts are persulfates, perborates, percarbonates and/or perphosphates, preferred cationic inorganic persalts are sodium, potassium and ammonium persulfate, sodium perborate monohydrate, sodium perborate tetrahydrate, sodium carbonate and their combinations.

7. The method as claimed in one or more of claims 1 to 6, wherein the oxidative breaker is selected from the group consisting organic peroxides, preferred organic peroxides are t-butyl peroxypivalat, dilauroyl peroxide, diacetyl peroxydicarbonate , dibenzoyl peroxide, t-amyl peroxybenzoate.

8. The method as claimed in one or more of claims 1 to 7, wherein the activator for oxidative breaker are N- acyl compounds which are able to react with hydrogen peroxide, preferably the N- acyl compounds are selected from the group of tetraacetyl ethylendiamine (TAED), tetraacetyl glycoluril, Ν,Ν'-diacetyl hydantoin and/or 1 ,5-diacetyl-2,4-dioxo hexahydro-1 ,3,5-triazine(DADHT).

9. The method as claimed in one or more of claims 1 to 8, wherein the activator for oxidative breaker are O-acyl compounds which are able to react with hydrogen peroxide, preferably the O- acyl compounds are selected from the group of acyloxy benzene sulfonic acids and their alkali metal salts, preferred from nonanoyloxy benzene sulfonic acid (NOBS), dodecanoyl oxybenzene sulfonic acid, benzoyloxy benzene sulfonic acid and/or decanoyl oxybenzoic acid and their alkali metal salts.

10. The method as claimed in claim 2, wherein the viscosified treatment fluid is injected at a temperature below 90°C, more preferred below 75°C.

1 1 . The method as claimed in one or more of claims 1 to 10, wherein the

viscosified treatment fluid containing at least a synthetic water soluble gelling agent contains multivalent metal ion or metal complexe selected from group IIIA, IVB, VB, VIB, IIVB and/or VIIIB of the periodic table of elements.

12. The method as claimed in claim 1 1 , wherein the multivalent metal ion or metal complexes is zirconium, aluminium, titanium, boron, chromium and/or iron, preferred zirconium and titanium.

13. The method as claimed in one or more of claims 1 to 12, wherein the

viscosified treatment fluid is injected as hydraulic fracturing fluid.

14. Method of stimulating a natural gas and/or oil reservoir comprising the

measures:

(iii) mixing said viscosified treatment fluid and said breaker composition and injected said mixture of viscosified treatment fluid and breaker composition into a natural gas or oil reservoir, preferably at a temperature of below 90°C,

(iv) allowing said viscosified treatment fluid and said breaker composition to interact whereby the viscosity of the viscosified treatment fluid is reduced,

(v) removing the viscosity reduced fluid from step (iv) from the natural gas or oil reservoir.

15. The method as claimed in claim 14, wherein the preferred embodiments are described in claims 2 to 13.

1 6. Method for the production of oil and/or natural gas which includes the

stimulation of the natural gas and/or oil reservoir by:

17. The method as claimed in claim 1 6, wherein the preferred embodiments are described in claims 2 to 13.